Added a missing header file to the header list
Added a missing header file to the header list

This became obvious when compiling against avogadrolibs.

Change-Id: I1bc71d09dabf8df2eafab8bf0a7fd51a8a65f433

cjh1/mongochemweb-avogadrolibs,wadejong/avogadrolibs,OpenChemistry/avogadrolibs,cjh1/mongochemweb-avogadrolibs,cjh1/mongochemweb-avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,wadejong/avogadrolibs,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,cjh1/mongochemweb-avogadrolibs,wadejong/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,wadejong/avogadrolibs